In a circuit board (hereinafter, described also as a “PCB”), a cable and a semiconductor integrated circuit (hereinafter, described also as an “LSI”) are mounted. It is desirable to design such a circuit board so as to have a low level of electromagnetic wave radiation (hereinafter, described also as “EMI” or “unnecessary electromagnetic wave radiation”) radiated from the circuit board and operate stably. For such a desire, a circuit board is configured and a layout is designed so as to allow an EMI amount generated from the circuit board to be at a low level.
The main cause of EMI is electromagnetic coupling between current flowing through a wiring of a circuit board and a cable connected to the circuit board. Such electromagnetic coupling causes common-mode current to flow through the cable and as a result, the cable acts as an antenna, resulting in occurrence of unnecessary electromagnetic wave radiation. Therefore, when current flowing through the wiring increases and electromagnetic coupling increases due to an increase in circuit operation speed and the like, the radiation amount of unnecessary electromagnetic waves also increases.
To suppress unnecessary electromagnetic wave radiation, it is necessary to take countermeasures for a configuration such as a layer structure, a layout, and the like of a circuit board, a characteristic of current flowing through a wiring, a length and connection location of a cable, addition of a countermeasure component, and the like. However, it is difficult to modify a design and add a countermeasure component for EMI suppression after production of a circuit board, and even when such countermeasures are taken, design cost and the like increases to a large extent.
Therefore, it is important to estimate an electrical characteristic in a design stage of a circuit board and take a countermeasure for EMI suppression from the result, as necessary. As a method for evaluating an unnecessary electromagnetic wave radiation amount in the design stage of the circuit board, a method for analyzing an electrical characteristic based on information of a board configuration and a mounted component is cited.
As a method for analyzing an electrical characteristic, there are cited an electromagnetic field analysis method such as an FDTD (Finite Difference Time Domain) method, a moment (MOM) method, a finite element (FEM) method, and the like and a circuit analysis method such as SPICE (Simulation Program with Integrated Circuit Emphasis) and the like.
However, it is difficult for SPICE to directly evaluate common-mode current and therefore, a special circuit model for causing current considered as common-mode current to flow is necessary. Further, to create the special circuit model, those with knowledge about electrical circuits and electromagnetic waves need to execute special processing, but it is difficult for those without in-depth knowledge to execute such processing. Further, even when such a special model is created, there is a risk in which a sufficient guarantee in terms of analysis accuracy is unobtainable. Therefore, it is very difficult to estimate common-mode current using SPICE.
On the other hand, in the electromagnetic field analysis method, the entire system to be a target is modeled and therefore, it is possible to calculate electromagnetic wave radiation based on common-mode current flowing through a cable. However, when the entire circuit board including a cable is modeled and a radiation electromagnetic field is calculated, enormous calculation cost is necessary. In general, there is a trade-off relation between calculation cost and analysis accuracy and therefore, when calculation cost is simply decreased, analysis accuracy is decreased, resulting in an insufficient guarantee for an analysis result.
From such a reason, there is needed a method in which to predict an unnecessary electromagnetic wave radiation amount in a design stage of a circuit board, a characteristic where common-mode current flowing through a cable is included can be predicted speedily with necessary analysis accuracy.
In addition, this method needs to be usable by those without in-depth knowledge about electrical circuits and electromagnetic waves.
As a technique for improving unnecessary electromagnetic wave radiation from a cable, various proposals have been made. For example, in Patent Literature 1, to speedily obtain a design guideline for suppressing EMI from a cable, an electronic device, a wiring, and a ground plane are converted into a model for electromagnetic field analysis from layout information of a circuit board and then a distribution of an electric field intensity generated in the vicinity of the ground plane associated with the operation of the electronic device is calculated. A portion where this electric field intensity is weak is connected with a cable and thereby, EMI from the cable is suppressed.
Further, Patent Literature 2 has proposed an electromagnetic field intensity calculation device including a model creation unit which creates a model of electrical circuit equipment for electromagnetic field intensity calculation by setting input data. There are provided a navigation file which stores a procedure for inputting external dimensions of the electrical circuit equipment and inputting an analysis frequency for analyzing the electrical circuit equipment via meshing and a display unit which sequentially displays the procedure stored in the navigation file; and in accordance with the procedure displayed on the display unit, a user sets input data conversationally.
Further, Patent Literature 3 has made a proposal for obtaining optimum analysis input data independent of the skill level of an input data creator, with respect to the electromagnetic field intensity dealt with in Patent Literature 2.